(1) Field of the Invention
The present invention relates to a semiconductor device and method of manufacture thereof.
More particularly, the invention relates to a semiconductor device in which one end of a bonding wire whose other end is connected to a bonding pad of a semiconductor element is in good connection to a lead frame and which displays excellent electrical properties in high temperature conditions or in high temperature, high humidity conditions.
(2) Description of the Prior Art
As shown in FIG. 1, in a conventional semiconductor device manufacturing method, a Ni plating layer 2 is formed on the surface of a lead electrode 1 made of copper, etc. and a bonding pad 5 on a semiconductor element 4 and the lead electrode 1 are connected via this Ni plating layer 2 by a bonding wire 3. In the drawing, a resin sealing body 6 and a solder layer 7 are formed via a Ni plating layer 9 on a lead frame 8.
However, with a device in which Ni plating layers 2 and 9 are formed, there are the problems that there is considerable variation in plating quality, that the manufacturing process for effecting plating treatment is complex and that the plating treatment makes manufacturing costs higher, etc. A practice in recent years, therefore, has been to effect direct connection of an aluminium bonding wire 3 onto a lead electrode 1 of copper, etc. without carrying out plating treatment.
However, in testing of semiconductor devices with aluminium bonding wires 3 connected directly to lead electrodes 1 of copper, etc. in long-time shelf tests in high temperature conditions (150.degree. C., 175.degree. C.) or in high temperature (about 80.degree. C.), high humidity (about 90%) conditions, there has been found to be a problem of reduced reliability because of opening at the bond interface between bonding wires 3 and lead electrodes 1.
A technique for resolving this problem in which bonding wire spanning between bonding wire constituted by copper wire and a lead frame made of copper is effected by selectively activating the bonding region is disclosed in Japanese Patent Application No. 55-88318. However, in this technique there are the problems that oxide forming on the bonding wire causes bond faults and that it is difficult to form a set ball at the bonding wire end portion. Further, working characteristics are poor because the bonding area has to the activated in each bonding process.
Japanese Patent Application No. 57-51237 discloses a bonding process technique in which a ball of required shape is formed and bonding oxidation is prevented by introducing a capillary end portion leading out from a bonding wire under a cover where a reducing atmosphere is maintained. However, this technique requires a complex structure including a cover for maintaining a reducing atmosphere and when the bonding process is effected at a process speed of one second or less there is the problem that maintenance and control are troublesome since faults occur easily. There is also the problem that it is not possible to effect highly reliable bonding between bonding wire constituted by copper wire and a lead frame made of copper since it is not possible to prevent oxidation on the lead frame side where an external lead is formed.